要使反射光线与入射光线的夹角为40°,则入射角应为 [ ]
A. 20°
B. 40°
C. 50°
D. 70°
答案:A
A11 of the boys_______soccer on the playground now. [ ]
A. playing
B. are play
C. are playing
D. plays
做工频耐压试验时,升压必须从零()开始,切不可冲击合闸。升压速度在()试验电压以前可以是任意的,以后的升压速度按每秒2%的速度均匀上升。